Proxy ARP

TomEastep

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2018/06/21

Overview

Proxy ARP (RFC 1027) is a way to make a machine physically located
on one network appear to be logically part of a different physical network
connected to the same router/firewall. Typically it allows us to hide a
machine with a public IP address on a private network behind a router, and
still have the machine appear to be on the public network "in front of"
the router. The router "proxys" ARP requests and all network traffic to
and from the hidden machine to make this fiction possible.

Consider a router with two interface cards, one connected to a
public network PUBNET and one connected to a private network PRIVNET. We
want to hide a server machine on the PRIVNET network but have it
accessible from the PUBNET network. The IP address of the server machine
lies in the PUBNET network, even though we are placing the machine on the
PRIVNET network behind the router.

By enabling proxy ARP on the router, any machine on the PUBNET
network that issues an ARP "who has" request for the server's MAC address
will get a proxy ARP reply from the router containing the router's MAC
address. This tells machines on the PUBNET network that they should be
sending packets destined for the server via the router. The router
forwards the packets from the machines on the PUBNET network to the server
on the PRIVNET network.

Similarly, when the server on the PRIVNET network issues a "who has"
request for any machines on the PUBNET network, the router provides its
own MAC address via proxy ARP. This tells the server to send packets for
machines on the PUBNET network via the router. The router forwards the
packets from the server on the PRIVNET network to the machines on the
PUBNET network.

The proxy ARP provided by the router allows the server on the
PRIVNETnetwork to appear to be on the PUBNET network. It lets the router
pass ARP requests and other network packets in both directions between the
server machine and the PUBNET network, making the server machine appear to
be connected to the PUBNET network even though it is on the PRIVNET
network hidden behind the router.

Example

The following figure represents a Proxy ARP environment.

Proxy ARP can be used to make the systems with addresses
130.252.100.18 and 130.252.100.19 appear to be on the upper
(130.252.100.*) subnet. Assuming that the upper firewall interface is eth0
and the lower interface is eth1, this is accomplished using the following
entries in /etc/shorewall/proxyarp:

Be sure that the internal systems
(130.242.100.18 and 130.252.100.19 in the above example) are not included
in any specification in /etc/shorewall/masq
(/etc/shorewall/snat on Shorewall 5.0.14 or later) or
/etc/shorewall/nat.

Note

The lower systems (130.252.100.18 and 130.252.100.19) should have their subnet mask and default gateway configured
exactly the same way that the Firewall system's eth0 is configured. In
other words, they should be configured just like they would be if they
were parallel to the firewall rather than behind it.

Warning

Do not add the Proxy ARP'ed address(es) (130.252.100.18 and
130.252.100.19 in the above example) to the external interface (eth0 in
this example) of the firewall.

Note

It should be stressed that entries in the proxyarp file do not
automatically enable traffic between the external network and the
internal host(s) — such traffic is still subject to your policies and
rules.

While the address given to the firewall interface is largely
irrelevant, one approach you can take is to make that address the same as
the address of your external interface!

In the diagram above, eth1
has been given the address 130.252.100.17, the same as
eth0. Note though that the VLSM is 32 so there is no
network associated with this address. This is the approach that I take with my DMZ.

To permit Internet hosts to connect to the local systems, you use
ACCEPT rules. For example, if you run a web server on 130.252.100.19 which
you have configured to be in the loc zone
then you would need this entry in /etc/shorewall/rules:

#ACTION SOURCE DEST PROTO DPORT
ACCEPT net loc:130.252.100.19 tcp 80

Warning

Your distribution's network configuration GUI may not be capable
of configuring a device in this way. It may complain about the duplicate
address or it may configure the address incorrectly. Here is what the
above configuration should look like when viewed using
ip (the line "inet 130.252.100.17/32 scope global
eth1" is the most important):

Note in particular that there is no broadcast address. Here is an
ifcfg-eth-id-00:a0:cc:d1:db:12 file from SUSE that
produces this result (Note: SUSE ties the configuration file to the card
by embedding the card's MAC address in the file name):

ARP cache

A word of warning is in order here. ISPs typically configure their
routers with a long ARP cache timeout. If you move a system from parallel
to your firewall to behind your firewall with Proxy ARP, it will probably
be HOURS before that system can
communicate with the Internet.

If you sniff traffic on the firewall's external interface, you can
see incoming traffic for the internal system(s) but the traffic is never
sent out the internal interface.

You can determine if your ISP's gateway ARP cache is stale using
ping and tcpdump. Suppose that we suspect that the gateway router has a
stale ARP cache entry for 130.252.100.19. On the firewall, run tcpdump as
follows:

tcpdump -nei eth0 icmp

Now from 130.252.100.19, ping the ISP's gateway (which we will
assume is 130.252.100.254):

Notice that the source MAC address in the echo request is different
from the destination MAC address in the echo reply!! In this case
0:4:e2:20:20:33 was the MAC of the firewall's eth0 NIC while
0:c0:a8:50:b2:57 was the MAC address of the system on the lower left. In
other words, the gateway's ARP cache still associates 130.252.100.19 with
the NIC in that system rather than with the firewall's eth0.

If you have this problem, there are a couple of things that you can
try:

A reading of TCP/IP Illustrated, Vol 1 by
Stevens reveals[1] that a “gratuitous” ARP packet should cause
the ISP's router to refresh their ARP cache (section 4.7). A
gratuitous ARP is simply a host requesting the MAC address for its own
IP; in addition to ensuring that the IP address isn't a
duplicate...

if the host sending the gratuitous ARP has just changed its
hardware address..., this packet causes any other host...that has an
entry in its cache for the old hardware address to update its ARP
cache entry accordingly.

Which is, of course, exactly what you want to do when you switch
a host from being exposed to the Internet to behind Shorewall using
proxy ARP (or one-to-one NAT for that matter). Happily enough, recent
versions of Redhat's iputils package include “arping”,
whose “-U” flag does just that:

It should be noted that IPv6 implements a "strong host model"
whereas Linux IPv4 implements a "weak host model". In the strong model, IP
addresses are associated with interfaces; in the weak model, they are
associated with the host. This is relevant with respect to Proxy NDP in
that a multi-homed Linux IPv6 host will only respond to neighbor
discoverey requests for IPv6 addresses configured on the interface
receiving the request. So if eth0 has address 2001:470:b:227::44/128 and
eth1 has address 2001:470:b:227::1/64 then in order for eth1 to respond to
neighbor discoverey requests for 2001:470:b:227::44, the following entry
in /etc/shorewall6/proxyndp is required: